There has heretofore been known a ball screw which includes a screw shaft, and a nut that is screwed to the screw shaft via plural rolling elements (e.g., balls) and in which the screw shaft and the nut are relatively rotatable. In this ball screw, frictional heat resulting from a point contact or surface contact is generated between the screw shaft and the nut during rotation. Hence, a cooling portion to reduce the frictional heat is provided in some cases.
Types of installing the cooling portion in the conventional ball screw include a shaft center cooling type and a nut cooling type. The shaft center cooling type is a configured such that the screw shaft is a cooling target so that the cooling portion is provided in the screw shaft. As an example of this shaft center cooling type, the screw shaft is hollow to allow a cooling medium to flow through the screw shaft. The nut cooling type is configured such that the nut is a cooling target and the cooling portion is provided in the nut.
Here, the shaft center cooling type may have a drawback in cost for making a hollow hole in the screw shaft, when the shaft center cooling type is used in a large and long ball screw device. Therefore, the nut cooling type is often used.
The technique disclosed in Patent Literature 1 is given as the ball screw using such a nut cooling type is. Specifically, according to the technique in Patent Literature 1, a cooling medium is made to pass through a flow channel provided in a nut in an axial direction to cool the nut.